The ductus arteriosus (DA) is dilated in the hypoxic environment of the developing fetus. At birth the DA constricts in response to normoxia. In contrast, the pulmonary artery (PA) is dilated in normoxia and constricts in hypoxia. Although their responses to oxygen tension (pO2) are opposite, both vascular tissues have O2-sensitive ion channels in their smooth muscle cells (SMC). We have shown that O2-sensitive K+ channels are partly responsible for these opposing effects of pO2 in these two smooth muscle ceil types. We have performed experiments indicating the existence of additional pO2-sensitive mechanisms in DA SMCs. Specifically, we hypothesize that 1. DASMCs, like PASMCs, have store-operated channels (SOCs) and they play an important role in normoxic vasoconstriction. 2. An acute rise in pO2 initiates calcium influx via SOCs in the DA while an acute fall in pO2 does the same in the PA. 3. Longer-term changes in pO2 alters the level of expression of SOCs in DA and PA SMCs before structural remodeling of vessels occurs. 4. Calcium sensitivity modulates the effect of pO2-mediated SMC cytosolic calcium changes on DA and PA vasoconstriction. Our studies will use molecular biology techniques including RNA interference to determine the identities of SOCs involved in pO2-mediated vasoconstriction in DA and PA. The effects of changes in pO2 on SOC activity in DA and PA smooth muscle cells will be determined using patch-clamp techniques. Changes in [Ca2+]i in response to altering pO2 will be recorded using single-cell Ca2+ imaging techniques. Calcium sensitivity will be assessed by measuring Rho/Rho-kinase activity and by performing b-escin-permeabilized DA and PA ring experiments to control [Ca2+]i. Persistent patent ductus is a common congenital defect in newborns and treatment of pulmonary hypertension is extremely limited. A better understanding of the mechanisms of O2-mediated changes in tone in the ductus arteriosus and pulmonary artery would therefore be of significant medical importance.